1. Field of the Invention
The invention generally relates to an oil supply system in an internal combustion engine comprising a valve operating device which is disposed in a cylinder head coupled to an upper surface of a cylinder block and which includes a valve operating cam shaft connected to a crank shaft rotatably carried in a lower engines body portion including the cylinder block, and valve operation characteristic changing means for changing, in accordance with a variation in hydraulic pressure in a hydraulic pressure chamber, the operation characteristic of an engine valve which is supported in the cylinder head for opening and closing.
2. Description of the Prior Art
Internal combustion engines including such a valve operating device are already known, for example, from Japanese Laid-open patent Application Nos. 229912/86 and 275516/86.
In such internal combustion engines, an oil pump for supplying a working oil to the hydraulic pressure chamber in the valve operation characteristic changing means is adapted to pump the working oil from an oil pan in a lower portion of an engine body. However, the oil pump is generally placed in the lower portion of the engine body and therefore, the distance between the oil pump and the hydraulic pressure chamber in valve operation characteristic changing means disposed in the cylinder heand, i.e., in an upper portion of the engine body is relatively long and hence, the supply of the oil to the hydraulic pressure chamber at the start of the engine is liable to be delayed.
Moreover, in general, an oil having a nature suitable for the lubrication of a crank shaft and a piston is used as an oil supplied from the oil pump. However, such oil has a large viscosity at a low temperature region, and the supply of an oil having a high viscosity to the hydraulic pressure chamber in the valve operation characteristic changing means in the valve operating device results in a non-smooth operation of the valve operation characteristic changing means and hence, the range of temperature for a normal operation of the valve operation characteristic changing means is limited. Thereupon, if an oil having a relatively low viscosity at a low temperature region is used, there is a fear of a seizure and a damage occurring in the crank shaft, the piston and the like.
In addition, in the lower portion of the engine body, the oil is exposed to blow-by gas and heated by heat of combustion and therefore, the deterioration of the oil progresses relatively rapidly. In contrast, in the upper portion of the engine body, there is no fear of contact of the oil with the blow-by gas, and the oil is less affected by heat of combustion, resulting in a relatively little increase in temperature of the oil. Nevertheless, if the same oil is used in the upper and lower portions of the engine body, it is necessary to replace all the oil at a relatively early cycle due to the deterioration of the nature of the oil due to the heating thereof in the lower portion of the engine body.
Accordingly, it is an object of the present invention to provide an oil supply system in an internal combustion engine, wherein the supply of an oil to the hydraulic pressure chamber at the start of the engine is conducted quickly, thereby providing an increase in range of temperature for the operation of the valve operation characteristic changing means, and also providing an improvement in life of the oil in the valve operating device.